Recently, wavelength division multiplexing (WDM) with which optical signals of different wavelengths are multiplexed for data transmission has been known as a communication method that realizes large volume data communications. In an optical communication system adopting the WDM, a wavelength selectable switch (WSS) is used in an optical transmission apparatus to multiplex optical signals of any wavelengths and transfer them to a targeted output destination. As the WSS, a type that has an one-port input and a multi-port output (drop-type WSS) and a type that has a multi-port input and a one-port output (add-type WSS) are known.
If the WSS is of the add type, an optical channel monitor (OCM) is incorporated together with the WSS in the optical transmission apparatus to detect the intensities of the optical signals output by the WSS in accordance with their wavelengths. The optical transmission apparatus controls the amount of attenuation for each wavelength at the WSS so that the intensity detected by the OCM for each wavelength reaches a target value.
Here, the structure of a conventional OCM is explained. FIG. 24 is a diagram for explaining an example of a conventional OCM. As illustrated in FIG. 24, when an optical signal obtained by multiplexing optical signals of different wavelengths is input, the conventional OCM collimates the input optical signal by the first optical system such as a lens, and guides it to a wavelength dividing unit. The wavelength dividing unit is provided with a wavelength tunable filter to allow an optical signal of a specific wavelength to pass through, and thereby divides the received optical signal into different wavelengths. For example, the wavelength dividing unit divides the received optical signal into wavelengths by controlling the temperature of the wavelength tunable filter and the incident angle of the optical signal with respect to the wavelength tunable filter and thereby changing the passing wavelengths at regular intervals of time. Then, an intensity detecting unit that is provided with a photo diode (PD) receives optical signals that are divided by the wavelength dividing unit, and detects the intensities of these optical signals in accordance with the wavelengths.
In the explanation of FIG. 24, the OCM that divides the optical signal into wavelengths by use of a wavelength tunable filter has been introduced, but an OCM that uses a diffraction grating in place of the wavelength tunable filter to divide an optical signal into wavelengths is also known. Furthermore, an OCM that uses a diffraction grating and a micro electro mechanical system (MEMS) mirror to divide an optical signal into wavelengths is also known.    Patent Document 1: Japanese Laid-open Patent Publication No. 2001-244557
However, multiple OCMs are often used together with the WSS in the optical transmission apparatus, and they are realized by space optical systems. It is therefore difficult to downsize an apparatus having multiple optical channel monitors. For example, a reconfigurable optical add/drop multiplexer (ROADM) incorporated in an optical transmission apparatus that is positioned at a node in the ring-type network is considered. In such a structure, an OCM is used with an add-type WSS for each of EAST-WEST and WEST-EAST, which increases the size of the apparatus.
Here, to downsize an apparatus having multiple OCM, a structure of combining the OCMs into one may be considered. For example, as illustrated in FIG. 25, the wavelength dividing units of two OCMs (OCM 1 and OCM 2) share a wavelength tunable filter so that two OCMs can be combined. FIG. 25 is a diagram for illustrating a structure in which two OCMs are combined.
However, a crosstalk problem that an optical signal passing through one OCM leaks into another OCM resides in the structure in which multiple OCMs are simply combined. In the example of FIG. 25, an optical signal that passes through the OCM 1 may leak into the OCM 2.